H.E.S.S.

High Energy Stereoscopic System

Gamma-rays from the BL Lac object RGB J0152+017

March 2008

The Automatic Telescope for Optical
Monitoring - is a robotic 75 cm telescope serving as an optical monitor of
variable gamma-ray sources, as a monitor of potential H.E.S.S. targets, and as a
instrument to measure atmospheric transmission. ATOM provided optical data
during the observations of the the BL Lac
object RGB J0152+017. The left image shows the telescope while in a dome in
Germany, the right image after installation on the H.E.S.S. site in Namibia.

The active galaxy RGB
J0152+017, at a redshift z=0.080, was selected as target for H.E.S.S.
observations because of its relative high radio and X-ray flux, characteristic
for TeV-emitting active galaxies (Fig. 1). "RGB" denotes
the Rosat/Green Bank catalog of objects detected in coincidence in Rosat X-ray
data and in the Green Bank 5 GHz radio catalog. H.E.S.S. observed the source
from Oct 30 to Nov 14, 2007 for a total of ~15 h live time. A preliminary
on-site analysis showed a significant gamma-ray excess from the direction of RGB
J0152+017, which was communicated by an Astronomer's
Telegram, alerting other observatories. Follow-up observations were
performed with the
ATOM optical
telescope on the H.E.S.S. site, with the
Nancay radio telescope, and by triggering target-of-opportunity observations
with the Swift
and RXTE X-ray
instruments. The offline analysis of H.E.S.S. data confirmed the detection (Fig.
2); the source has a flux of about 2% of the flux from the Crab nebula and a
spectrum with photon index of approximately 3.

Modeling of the wide-band emission spectra of active galaxies is complicated due
to the high variability of these objects, which on the one hand requires
simultaneous observation in the different wavelength regimes of interest, and
which on the other hand do not permit the use of steady-state solutions. In case
of RGB J0152+01, virtually simultaneous data
was recorded in the radio, optical, X-ray and gamma-ray domains. In none of
these, significant variability is seen, implying that the source was in a
steady, non-flaring state. The measured energy output across the
electronmagnetic spectrum is shown in Fig. 3. It can be
interpreted in a SSC model (Katarzynski
et al. 2001), where a rapidly moving compact blob of high-energy electrons
inside the AGN jet emits synchrotron radiation and creates, by upscattering of
sychrotron quanta by electrons, gamma rays, forming a double-humped spectrum.
Optical frequencies receive a dominant contribution of thermal radiation from
the host galaxy; an additional extended sychrotron source needs to be postulated
to explain the radio data.

References:

Fig. 1:
X-ray flux versus radio flux for active galaxies, from
Costamante and
Ghisellini (2002). Candidate VHE gamma-ray emitters are objects in the
top right corner, combining high radio and X-ray flux. Objects detected in
VHE gamma rays are shown by the black circles. RGB J0152+017 is shown as a red cross.

Fig. 2:
Smoothed gamma-ray sky map of the region around RGB J0152+017; the color scale gives the gamma-ray
flux, the black contours the 3, 4, 5 sigma significance levels. The star
denotes the position of RGB J0152+017. The observed excess is
consistent with a point source, the width of the distribution being
compatible with the angular resolution of the instrument, has a significance
of 6.6 standard deviations and the position is within the 20" errors
consistent with the location of RGB J0152+017.

Fig. 3:
Spectral energy distribution of radiation from RGB J0152+017; colored symbols refer to
quasi-simultaneous observations by H.E.S.S., by the Swift and RXTE X-ray
satellites, by the ATOM optical telescope and the Nancay radio telescope.
The insets give expanded views of the X-ray and gamma-ray spectra. The solid
line represents an SSC model where X-rays and gamma rays represent synchrotron radiation and inverse Compton
scattering of electrons confined in a volume of about 1015 cm,
moving at highly relativistic speed. An additional syncrotron component from
a more extended region is added to explain the radio flux.